Audit url/devel/head for redundant :using

Library/Homebrew/cmd/audit.rb

@@ -294,6 +294,16 @@ class FormulaAuditor
         problem "#{cksum.hash_type} should be lowercase" unless cksum.hexdigest == cksum.hexdigest.downcase
       end
     end
+
+    # Check for :using that is already detected from the url
+    @specs.each do |s|
+      next if s.using.nil?
+
+      url_strategy = DownloadStrategyDetector.detect(s.url)
+      using_strategy = DownloadStrategyDetector.detect('', s.using)
+
+      problem "redundant :using specification in url or head" if url_strategy == using_strategy
+    end
   end
 
   def audit_patches

Library/Homebrew/formula_support.rb

@@ -4,6 +4,7 @@ require 'version'
 
 class SoftwareSpec
   attr_reader :checksum, :mirrors, :specs
+  attr_reader :using # for auditing
 
   def initialize url=nil, version=nil
     @url = url